Practice question · Estimate
A flash is moved from 2 metres to 4 metres from its subject. How many stops of light are lost?
Estimate on a scale from 0 stops (0) to 5 stops (5).
Hints
- Illuminance falls as one over the square of the distance.
- A quarter of the light is two halvings.
Show the answer
2 (answers within ±0.4 count)
Why
Two stops. Doubling the distance quarters the illuminance, because the same light is spread over four times the area. This is why moving a light a little in a small studio changes the exposure so much, and why a flash on a camera is so unforgiving of subjects at different distances.
Practise Exposure and Metering
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